Although present-day deflection yokes produce a self-convergence of the three beams in a cathode-ray tube, the price paid for such self-convergence is a deterioration of the individual electron beam spot shapes. The yoke magnetic field is astigmatic, and it both overfocuses the vertical-plane electron beam rays, leading to deflected spots with appreciable vertical flare, and underfocuses the horizontal rays, leading to slightly enlarged spot width. To compensate, it has been the practice to introduce an astigmatism into the beam-forming region of the electron gun to produce a defocusing of the vertical rays and a focusing of the horizontal rays. Such astigmatic beam-forming regions have been constructed by means of G1 control grids or G2 screen grids having slot-shaped apertures. These slot-shaped apertures produce non-axially-symmetric fields with quadrupolar components which act differently upon rays in the vertical and horizontal planes. Such slot-shaped apertures are shown in U.S. Pat. No. 4,234,814, issued to Chen et al. on Nov. 18, 1980. These constructions are static; the quadrupole field produces the same compensatory astigmatism even when the beams are undeflected and experiencing no yoke astigmatism.
To provide improved correction, U.S. Pat. No. 4,319,163, issued to Chen on Mar. 9, 1982, introduced an extra upstream screen grid, G2a, with horizontally slotted apertures, and with a variable or modulated potential applied to it. The downstream screen grid, G2b, has round apertures and is at a fixed potential. The variable voltage on G2a varies the strength of the quadrupole field, so that the astigmatism produced is proportional to the scanned off-axis position.
Although effective, use of astigmatic beam-forming regions has several disadvantages. First, beam-forming regions have a high sensitivity to construction tolerances because of the small dimensions involved. Second, the effective length or thickness of the G2 grid must be changed from the optimum value it has in the absence of slotted apertures. Third, beam current may vary with a variable potential applied to a beam-forming region grid Fourth, the effectiveness of the quadrupole field varies with the position of the beam cross-over and, therefore, with beam current. Therefore, it is desirable to develop astigmatism correction in an electron gun which is not subject to all these disadvantages.